Due to the remarkable advances made in the field of medicine in recent years, opportunities for saving lives are continuing to increase by eliminating the causes of diseases through, for example, systematic therapeutic techniques in the form of the surgical excision of cancer or living donor transplant techniques for tissues and organs. However, there are cases in which patients suffer a considerable decrease in QOL accompanying excision of an affected area. In addition, there are limitations on treatment dependent upon living donor transplants due to such factors as a shortage of transplant donors and the occurrence of rejection. If it were possible to regenerate a tissue or organ that has been lost due to surgical treatment or an unforeseen accident, then it would be possible to considerably improve the QOL of patients. In addition, regenerative medicine also makes it possible to resolve the problems confronting living donor transplants. From this viewpoint, the degree of expectations being placed on regenerative medicine is high.
Technologies in which regenerative medicine has been successful are primarily related to comparatively simple tissue in terms of morphology or function in the manner of artificial skin, artificial bone and artificial teeth. Reconstructed artificial skin and artificial bone is incorporated into cells enabling the providing of signals required for tissue construction. However, there have been limitations on the repertoire of differentiation of artificial skin and artificial bone by regenerative medicine techniques. For example, although allogenic keratinocytes or skin fibroblasts and the like differentiate into structures in the form of the epidermis, are incorporated by surrounding organs to eventually have a horny layer or basal layer having barrier properties, there has been reported to be no derivation of secondary derivatives such as hair follicles, sebaceous glands or sweat glands.
Body tissue normally contains both cells that are able to self-replicate and possess stem cell properties for maintaining tissue homeostasis by sending signals to differentiated cells or supplying differentiated cells, and cells having properties of somatic cells that have already differentiated that receive various signals or commands from such cells, and is able to function through interaction between both of these types of cells. In the case of vertebrates, for example, interaction between mesenchymal cells and epithelial cells is essential for nearly all tissue and organ formation. In the case of hair follicles, mesenchymal cells in the form of hair papilla cells are responsible for stem cell-like properties, while epithelial cells in the form of keratinocytes are equivalent to cells having somatic cell-like properties in the sense that they differentiate into hair shafts (hair itself).
The difficulty encountered when forming organs by regenerative medicine lies in reaching a state of coexistence between cells having stem cell-like properties maintained in an undifferentiated state and cells that have already differentiated as in actual body tissue. In the prior art, even if epithelial cells and mesenchymal cells were able to be co-cultured, they either both ended up differentiating or both maintained an undifferentiated state, thereby preventing the reproduction of the coexistence of undifferentiated cells and differentiated cells so as to mimic actual body tissue.